The Tremarctinae or short-faced bears is a subfamily of Ursidae that contains one living representative, the spectacled bear ( Tremarctos ornatus) of South America, and several extinct species from four genera: the Florida spectacled bear ( Tremarctos floridanus), the North American giant short-faced bears Arctodus ( A. pristinus and A. simus), the South American giant short-faced bear Arctotherium (including A. angustidens, A. vetustum, A. bonariense, A. wingei, and A. tarijense) as well as Plionarctos (P. edensis and P. harroldorum), which is thought to be ancestral to the other three genera.
Taxonomy
Tremarctinae was erected by John Merriam and
Chester Stock in 1925.
Traditionally, analyses of the phylogenetic inner relationships of tremarctines had
Plionarctos and
Tremarctos as basal groups with respect to a short-faced bear clade of
Arctodus and
Arctotherium.
A study of the affinities of bears belonging to
Arctotherium indicates that they were more closely related to the
spectacled bear than to
Arctodus, implying convergent evolution of large size in the two lineages.
Tremarctines are occasionally referred to as
arctodonts and/or
arctotheres,
or
tremarctotheres in older scientific literature.
Diagnostics
Skull
Tremarctinae appear to have a disproportionately shorter snouts compared with most modern bears, giving them the name "short-faced." This apparent shortness is an illusion caused by the deep snouts and short nasal bones of tremarctine bears compared with
Ursinae bears; Tremarctinae had shorter and taller skulls, but not a shorter face than most living bears.
In addition to being
Brachycephaly, tremarctine bears' skulls possessed well developed
and
Mandibular fossa,
a premasseteric fossa on the
Mandibular fossa (except for
Plionarctos),
and often an entepicondylar foramen on the
humerus of tremarctine bears.
Moreover, tremarctine bears' orbits are also bigger, more rounded and lateralized than ursine bears.
The premassateric fossa is used to detect maturity in tremarctine bears, with only adults possessing fully developed fossa.
The features of tremarctine bear's individual morphology, such as dentition, can be quite variable (particularly the M2 molar).
Unlike tremarctine bears, ursine bears have only one masseteric fossa on their mandible and more slender and elongated skulls, with generally narrower molars (with the exception of polar bears).
Osteological differences between tremarctine and
Ursinae bears also include an extra lateral cusp between the
trigonid and
talonid on the m1 molar.
Postcranial
The
ichnotaxon Ursichnus sudamericanus is referred to
Arctotherium tarijense; unlike the footprints of ursine bears, the heel impression is absent in the manus, and the digit III imprint is longer than the remaining digits.
Being very similar to extant spectacled bears,
Ursichnus sudamericanus suggests all tremarctines had long claws and five digits whose tips arrayed in a shallow arc.
While all are plantigrade, Tremarctinaes can be distinguished between short-footed (
Tremarctos) and long-footed forms (
Arctodus).
Evolution
originate with their common ancestor,
Plionarctos, in the
Hemphillian (
Late Miocene, ~7Ma) of North America;
Plionarctos is last recorded in the
Blancan (Pliocene, ~3Ma).
Around the
Late Miocene-
Zanclean boundary (~5.3 Ma), tremarctines, along with other
Bear, experienced an explosive radiation in diversity, as C4 vegetation (
Poaceae) and open habitats dominated, the world experienced a major temperature drop and increased seasonality, and a faunal turnover which extinguished 60–70% of all Eurasian faunal genera, and 70–80% of North American genera.
Correspondingly, genetic studies suggest that the mean divergence dates for
Arctotherium,
Arctodus and
Tremarctos were between 5.5Ma and 4.8Ma,
and between
Arctotherium and
Tremarctos at 4.1 Ma.
All three genera are first recorded from the
Blancan (Pliocene/Pleistocene boundary) of North America.
An indeterminate Arctotherium is first recorded from the late Pliocene of El Salvador.
By the terminal Pleistocene, Arctodus simus, Tremarctos floridanus, Arctotherium tarijense and Arctotherium wingei collectively occupied a range from Alaska to southernmost Patagonia. All of these forms were extinct by the end of the early Holocene.
Paleobiology
Tremarctines inhabited a wide range of niches- from small and mostly herbivorous bears inhabiting more forested habitat, such as
Arctotherium and
Spectacled bear, to the colossal
Arctotherium and
Arctodus; plains adapted omnivores with a penchant for large quantities of meat. Although the two giant species appear superficially similar, both species had key, significant differences. While
Arctodus had a wide range across North America for 800,000 years,
Arctotherium appears to be limited to the
Southern Cone, in open plains habitat. Furthermore, whereas
Arctodus simus varied its diet between quasi-carnivory in Alaska to classic omnivory,
Arctotherium had similar rates of carnivory across specimens, according to isotope studies.
Additionally, the much more gracile form of
Arctodus, in contrast with the robust
Arctotherium, has puzzled researchers.
However, it has been posited that the
Blancan extinctions of scavenger-niche mega-carnivores, such as the
Procyonidae Chapalmalania in South America, and both
Borophagus and
Agriotherium in North America, was a shared impetus for gigantism in
Arctodus and
Arctotherium.
Arctodus and Tremarctos share characteristics common to herbivorous bears. This includes cheek teeth with large surface areas, a deep mandible, and large mandibular muscle attachments. Because herbivorous carnivorans lack an efficient digestive tract for breaking down plant matter via microbial action, they must break down plant matter via extensive chewing or grinding, and thus possess features to create a high mechanical advantage of the jaw. This presents the possibility that these traits may be an ancestral condition of the group, if not an indication of their preferred dietary habits.
Systematics
The following taxonomy of the tremarctine bears follow by Mitchell et al. (2016):
-
Subfamily Tremarctinae Merriam & Stock, 1925
-
† Plionarctos Frick, 1926
-
† Plionarctos edensis Frick, 1926
-
† Plionarctos harroldorum Tedford & Martin, 2001
-
† Arctodus Leidy, 1854
-
† Arctodus pristinus Leidy, 1854
-
† Arctodus simus Cope, 1879
-
† Arctotherium Burmeister, 1879
-
† Arctotherium angustidens Gervais & Ameghino, 1880
-
† Arctotherium bonariense Gervais, 1852
-
† Arctotherium tarijense Ameghino, 1902
-
† Arctotherium vetustum Ameghino, 1885
-
† Arctotherium wingei Ameghino, 1902
-
Tremarctos Gervais, 1855
-
† Tremarctos floridanus (Gildey, 1928)
-
Tremarctos ornatus (Cuvier, 1825) – spectacled bear
See also
-
Pleistocene megafauna
-
Pleistocene extinctions
External links
